Abstract
BACKGROUND: Versions of the Health Assessment Questionnaire (HAQ) are commonly used to measure physical functioning across multiple rheumatic diseases but there has been no clear demonstration that any HAQ version is actually generic. This study aimed to show that the HAQ-II instrument is invariant across different rheumatic disease categories using the Rasch measurement model, which would confirm that the instrument is generic. METHODS: HAQ-II responses from 882 consecutive rheumatology clinic attendees were fitted to a Rasch model. Invariance across disease was assessed by analysis of variance of residuals implemented in RUMM2030. Rasch modeled HAQ-II scores across disease categories were compared and the mathematical relationship between raw HAQ-II scores and Rasch modeled scores was also determined. RESULTS: The HAQ-II responses fitted the Rasch model. There was no substantive evidence for lack of invariance by disease category except for a single item ("opening car doors"). Rasch modeled scores could be accurately obtained from raw scores with a cubic formula (R(2) 0.99). Patients with rheumatoid arthritis had more disability than patients with other kinds of inflammatory arthritis or autoimmune connective tissue disease. CONCLUSIONS: The HAQ-II can be used across different rheumatic diseases and scores can be similarly interpreted from patients with different diseases. Transforming raw scores to Rasch modeled scores enable a strictly linear, interval scale to be used. It remains to be seen how that would affect interpretation of change scores. TRIAL REGISTRATION: ANZCTR ACTRN12617001500347 . Registered 24th October 2017 (retrospectively registered).